From e2ea152b4ba503a9f8b8d6d3f33f23f82a94a996 Mon Sep 17 00:00:00 2001
From: Simon Byrne <simonbyrne@gmail.com>
Date: Tue, 29 Mar 2022 00:35:02 -0700
Subject: [PATCH] Allow pushing to vector after 3-arg ldiv! (#43510)

Co-authored-by: Daniel Karrasch <daniel.karrasch@posteo.de>
---
 stdlib/LinearAlgebra/src/factorization.jl | 17 ++++++++++++++---
 stdlib/LinearAlgebra/test/lu.jl           |  9 +++++++++
 stdlib/LinearAlgebra/test/qr.jl           |  8 ++++++++
 3 files changed, 31 insertions(+), 3 deletions(-)

diff --git a/stdlib/LinearAlgebra/src/factorization.jl b/stdlib/LinearAlgebra/src/factorization.jl
index 626a1ae7b1a74..bfaffd0dccd14 100644
--- a/stdlib/LinearAlgebra/src/factorization.jl
+++ b/stdlib/LinearAlgebra/src/factorization.jl
@@ -114,8 +114,18 @@ end
 /(B::TransposeAbsVec, adjF::Adjoint{<:Any,<:Factorization}) = adjoint(adjF.parent \ adjoint(B))
 
 
-# support the same 3-arg idiom as in our other in-place A_*_B functions:
-function ldiv!(Y::AbstractVecOrMat, A::Factorization, B::AbstractVecOrMat)
+function ldiv!(Y::AbstractVector, A::Factorization, B::AbstractVector)
+    require_one_based_indexing(Y, B)
+    m, n = size(A, 1), size(A, 2)
+    if m > n
+        Bc = copy(B)
+        ldiv!(A, Bc)
+        return copyto!(Y, 1, Bc, 1, n)
+    else
+        return ldiv!(A, copyto!(Y, B))
+    end
+end
+function ldiv!(Y::AbstractMatrix, A::Factorization, B::AbstractMatrix)
     require_one_based_indexing(Y, B)
     m, n = size(A, 1), size(A, 2)
     if m > n
@@ -123,7 +133,8 @@ function ldiv!(Y::AbstractVecOrMat, A::Factorization, B::AbstractVecOrMat)
         ldiv!(A, Bc)
         return copyto!(Y, view(Bc, 1:n, :))
     else
-        return ldiv!(A, copyto!(Y, view(B, 1:m, :)))
+        copyto!(view(Y, 1:m, :), view(B, 1:m, :))
+        return ldiv!(A, Y)
     end
 end
 
diff --git a/stdlib/LinearAlgebra/test/lu.jl b/stdlib/LinearAlgebra/test/lu.jl
index f07ceceec8444..e86cd583c0904 100644
--- a/stdlib/LinearAlgebra/test/lu.jl
+++ b/stdlib/LinearAlgebra/test/lu.jl
@@ -426,4 +426,13 @@ end
     end
 end
 
+@testset "can push to vector after 3-arg ldiv! (#43507)" begin
+    u = rand(3)
+    A = rand(3,3)
+    b = rand(3)
+    ldiv!(u,lu(A),b)
+    push!(b,4.0)
+    @test length(b) == 4
+end
+
 end # module TestLU
diff --git a/stdlib/LinearAlgebra/test/qr.jl b/stdlib/LinearAlgebra/test/qr.jl
index a7b24f08385f2..d5f5537b2f63f 100644
--- a/stdlib/LinearAlgebra/test/qr.jl
+++ b/stdlib/LinearAlgebra/test/qr.jl
@@ -270,11 +270,19 @@ end
     b  = randn(3)
     b0 = copy(b)
     c  = randn(2)
+    B  = randn(3,3)
+    B0 = copy(B)
+    C  = randn(2,3)
     @test A \b ≈ ldiv!(c, qr(A ), b)
     @test b == b0
+    @test A \B ≈ ldiv!(C, qr(A ), B)
+    @test B == B0
     c0 = copy(c)
+    C0 = copy(C)
     @test Ac\c ≈ ldiv!(b, qr(Ac, ColumnNorm()), c)
     @test c0 == c
+    @test Ac\C ≈ ldiv!(B, qr(Ac, ColumnNorm()), C)
+    @test C0 == C
 end
 
 @testset "Issue reflector of zero-length vector" begin